1. Field of the Invention
The present invention relates to a movable magnet type linear motor and further to a linear compressor.
2. Description of the Related Art
In recent years, linear motors have been in active development. Heretofore, in Europe and America, a linear motor has been studied for use in a Stirling engine to be used in the outer space. In the late years, the American SUNPOWER Co., Ltd. has developed a linear compressor and a movable magnet linear motor for a compressor to be used under common environments (Nicholas R. van der Walt, Reuven Unger: Linear Compressors-amturing technology, International Appliance Technical Conference, pp1-6, 1994).
FIG. 37 is a schematic illustration of a conventional linear motor.
The linear motor, generally designated at numeral 300, is made up of a cylindrical inner yoke 301, an outer yoke 304 including two magnetic pole portions 302, 303, a coil 306 wound around a central axis 305 and a moving element 308 having a cylindrical permanent magnet 307.
In this configuration, on supply of an alternating current to the coil 306, different magnetic poles alternately take place axially in the magnetic pole portions 302, 303, and the magnetic attraction and repulsive action with the permanent magnet 307 of the moving element 308 generate a thrust proportional to the magnitude of the current in the coil 306 and the magnetic flux density in the permanent magnet 307, so that the moving element 308 reciprocates in synchronism with the frequency of the alternating current.
In the aforesaid conventional example, the inner yoke 301 and the outer yoke 304 are constructed in a manner that a large number of sheets (thin plates) each having a high magnetic permeability are piled up each other. The eddy current loss of the motor shows a property proportional to the square of the plate thickness of the yoke material, and therefore, such a configuration as seen in the conventional example is capable of reducing the eddy current loss to improve the lowering of the motor efficiency due to the core (iron) loss, as compared with the case that the yoke is merely formed with a metallic block.
There is a problem which arises with the configuration of the conventional linear motor, however, in that, since the yoke has a cylindrical shape to make a dimensional difference between its inner circumferential section and its outer circumferential section, considerable difficulty is encountered in accurately piling up sheets having an even thickness toward its central axis at the fabrication. For this reason, the sheets have been made not to have an even thickness, but the outer circumferential section has been designed to have a slightly greater thickness to form it into a wedge-like shape. However, this contributes to an extremely high manufacturing cost.
Furthermore, FIG. 38 is a cross-sectional view showing a construction of a conventional linear compressor. In FIG. 38, a linear compressor, designated at numeral 400, is composed of a cylinder 401, a piston 402 inserted into the cylinder 401 to be allowed to reciprocate therein, a compression chamber 403 defined in a state of facing a head of the piston 402, and a suction (inlet) valve (not shown) and delivery (outlet) valve (not shown) openable and closable in response to a gas pressure in the compression chamber 403.
The linear compressor 400 is additionally equipped with a linear motor 406 for making the piston 402 reciprocate and a resonance spring 407 for supporting the piston 402 to allow the reciprocation of the piston 402. The linear motor 406 comprises a cylindrical inner yoke 408, an outer yoke 411 including two magnetic pole portions 409, 410, a coil 413 and a moving element 415 having a cylindrical permanent magnet 414, with the moving element 415 being in connection with the piston 402.
On supply of an alternating current to the coil 413, different magnetic poles alternately take place axially in the magnetic pole portions 409, 410, the magnetic attraction and repulsive action with the permanent magnet 414 of the moving element 415 develops a thrust proportional to the magnitude of the current in the coil 413 and the magnetic flux density of the permanent magnet 414, so that the moving element 415 reciprocates in synchronism with the frequency of the alternating current, and consequently, the piston 402 also reciprocates. Further, when the interior of the compression chamber 403 assumes a low pressure condition, an expanded gas is taken through the suction valve into the compression chamber 403, while, when assuming a high pressure condition, a compressed gas is discharged from the compression chamber 403 through the delivery valve, thus serving as a compressor.
The core (iron) loss such as an eddy current loss and a hysteresis loss hinders the improvement of a motor and a compressor. Since the eddy current loss is proportional to the square of the thickness of the yoke material, it is effective that a yoke is constructed by piling up sheets. However, as mentioned above, the conventional linear motor or the linear motor of the conventional linear compressor is composed of a yoke having a cylindrical configuration, and this cylindrically configured yoke encounters considerable difficulty in accurately piling up sheets toward its central axis at its fabrication.